I descibe two different attacking techniques that help maximize your damage output and then list tactics to use in each dungeon for defeating the bosses.
When facing bosses, it can be hard to know what is the best weapon to use. Also some bosses are invulnerable to certain weapons or can only be hurt at specific times. This guide will try to not reveal anything about the game's storyline or what the bosses actually are so that you can still discover them for yourself.
The main attack techniques that you should master before fighting the bosses is shooting both the crossbow and shotgun at the same time (I call it bow-blasting). This technique will allow you to do the maximum damage in the early game. Later you will be able to find a second shotgun and after fully upgrading your guns attack you will do considerable amounts of damage to any enemy that is in your path.
In the later game it is very effective to do bow-blasting and while your gun and crossbow is reloading quickly switch to the second shotgun and fire it too. Then while the second gun is reloading switch back to bow-blasting and just repeat until your target is dead. (I don't know what to call this technique yet, but for now I will refer to it as the blast-n-switch)
Dungeon 1 - if you have the first upgrade for the crossbow you can usually kill it with 6-7 combined shots. Just make sure to shoot then move so that the blue ball doesn't hit you or absorb the shot in place of the boss. The blue balls will spawn skeletons. For the first boss you can ignore the skeletons if you kill it fast enough then clean them up afterwards.
Dungeon 2 - This dungeon has multiple bosses that you must kill. Just constantly shoot and move while deploying dynamite and crossbow shots as the enemies line up and you shouldn't have a problem.
Dungeon 3 - Your first real boss. You can only damage it when it has it's mouth open. It's four arms will come at you but will only target where you were standing when they started moving. So try moving closer to the boss and staying a few spaces up from the bottom of the screen until one of the arms move. then pull back and/or move side to side to dodge the arms. The mouth will typically open after two waves of attacks from the arms. So try to be in front of the mouth after the second wave to maximize your chance of a hit and remember to use the bow-blasting attack as much as possible. Also look out for blue balls that will come from the boss's mouth when it opens. When the mouth is closed clear out any skeletons that have spawned but save your crossbow ammo for the boss. If you don't kill the skeletons quickly you will become overwhelmed and die.
Dungeon 4 - This one can get tricky becaus the boss will shoot at you and you can't dodge the shot after the boss fires. For this boss it is best to use the natural cover to hide and then pop out after each shot to then attack the enemy with whatever you have ammo for. The wave cannon can let you damage it while behind cover, but you will still need to leave cover to do some bow-blasting to finish the boss off. The boss has a nasty habit of pursuing you behind your cover so sometimes it is effective to leave a stick of dynamite just before you run for the next piece of cover as a surprise.
Dungeon 5 - This room has multiple bosses again but this time they are faster and launch projectiles at you. So just like the last dungeon use the cover to your advantage.
Dungeon 6 - You will face a wonderful big boss that likes to disappear while also throwing projectiles at you that can richochte off walls. This time doing the bow-blasting will probably result in you wasting ammo. So first shoot your gun and when you hit the boss it will reappear momentarily. When you see it unload with your crossbow and gun. If you make most of your shots and you have been gathering the upgrades for your gun you should be able to finish off the boss before needing to get health from the cooler or even use the shield to block the projectiles. The wave cannon is useless here.
Dungeon 7 - You will probably recognize this boss from the promotion video on the store page. The easist I have found to defeat it is to just throw dynamite at its tail while running circles around it.
Dungeon 8 - Now for two bosses at the same time. This will be a pretty straight forward boss fight. Shoot at them when you can and run away when they leave their little "presents". I have noticed that they don't ever enter the two squares directly in front of the entrance door. So if you need to dodge them to get your bearings, go there.
Dungeon 9 - Another big boss. This time though in addition to only being able to damage it while it's mouth is open, the boss also has a shield which can block your shots. The way to fight it is similar to the boss of dungeon 3 except that you should move left towards the end of the second round of arm swinging so that the mouth is more exposed.
Dungeon 10 - This dungeon has several mini bosses and one final big boss. For the mini bosses you will meet a combination of the previous dungeon bosses and a few new ones. At this time dungeon 10 is not yet finalized so I don't want to give too much information and then have it be wrong due to an update of the game. I suggest doing lots of blast-n-switch while at the same time running for your life. On the final boss, be prepared to run for cover everytime you see its eye charging and kill the spawning monsters almost as soon as they spawn. When the boss gets low in health it will perform its eye attack more rapidly so be quick to run and hide.